This invention relates generally to an engine for a motor vehicle and more particularly to a system for controlling an ignition timing of the engine. More specifically, the invention relates to an ignition timing control system of a time-control type which sets the ignition timing in accordance with a time interval from a crank angle reference signal or from a reference crank angle position.
Heretofore, among ignition timing control systems of the time-control type, there has been a system which utilizes signals of crank angles of even pitch, and in which, in order to improve the reference position detection accuracy, the ignition timing is corrected on the basis of the difference between a value derived by averaging processing of an engine rotational speed and the engine rotational speed thereby to determine an appropriate ignition advance angle as indicated, for example, in Japanese Patent Laid-Open Publication No. 6071/1985 (corresponding to U.S. Pat. No. 4,498,438), and thereby to reduce a car knocking (bucking or hiccupping motion) of the vehicle due to misfiring.
Furthermore, as disclosed in Japanese Patent Laid-Open Publication No. 32974/1985, there has been a system in which, by computing the ignition timing through the use of engine rotational speed data, which has been averaged, when a specific time has elapsed after starting of the engine and the engine speed is at a specific value or higher than the specific value, excessively advancing of the ignition angle occurring in accordance with increasing of the engine speed is prevented, and deviation in the ignition timing at high speed is reduced.
Another problem which is not solved in the systems of the above cited references is that the crank angular velocity varies even during one cycle of combustion because of the generation of torque due to combustion and absorption of torque due to compression, this crank angular velocity variation being large particularly under conditions such as those at the time of starting by cranking, and, by method of detecting the crank angular velocity as described in the above mentioned references, there is the possibility of a difference occurring between the set advance angle value and an actual ignition timing.
Ordinarily, the ignition timing in terms of the crank angle, is stored beforehand in a map stored in a microcomputer as a value determined by the engine operational conditions such as an engine speed and an intake air flow rate. In the case where the ignition timing is controlled by the time interval from a datum reference crank angle position signal, the time interval from the reference crank angle position to the ignition timing is calculated from the value of the angle derived from the above mentioned map and the crank angular velocity at the time, and, on the basis of this time interval, ignition timing control is carried out. Ignition timing control according to a time interval control method of this character can be carried out continuously with an extremely high resolution in the normal state.
In the case of ignition timing control depending on the time interval from the reference crank angle signal detected, it is necessary to convert crank angle into time interval. For this purpose, the rotational speed of the crank, that is, the engine speed, must be detected. Then, in terms of the ignition timing which has been set (ignition advance angle BTDC) A.sub.ADV, the engine rotational speed N, the reference crank angle A.sub.REF ' and a proportional constant .alpha., the time interval T.sub.ADV from the reference crank angle signal detected to the ignition is expressed as follows. EQU T.sub.ADV =.alpha..times.1/N .times.(A.sub.REF -A.sub.ADV)
In order to calculate this engine rotational speed N, it is necessary to measure the time interval T.sub.REF between a reference crank angle detected A.sub.REF and reference crank angle detected B.sub.REF adjacently provided at the advancing side in the ignition timing of the angle A.sub.REF. However the crank angular velocity varies, even during one cycle, because of increase of torque due to combustion and decrease of torque due to compression, whereby the value of the engine rotational speed N changes depending on the angular position at which the time interval T.sub.REF between the reference crank angles is measured.
Consequently, if the time interval T.sub.REF is measured at a position where the crank angular velocity is high within one cycle of combustion, the ignition position A.sub.ADV tend to be offset in the advance direction, whereas if the time interval T.sub.REF is measured at a position where the crank angular velocity is low within one cycle of combustion, the ignition position A.sub.ADV tends to be offset in the retardation direction. This tendency is pronounced particularly in the low-speed region. Furthermore, if this measurement is carried out before one cycle of combustion, a delay in the response of the engine speed N with respect to the ignition timing T.sub.ADV occurs at the engine rotation in a transient state. As a consequence, at the transient operation such as that in the low-speed rotation as at the starting of the engine or of the vehicle, ignition timing cannot be controlled accurately and positively.